A variety of sensors are known that use an electrochemical cell to provide output signals by which the presence or absence of an analyte in a sample can be determined. For example in an electrochemical cell, an analyte (or a species derived from it) that is electro-active generates a detectable signal at an electrode, and this signal can be used to detect or measure the presence and/or amount within a biological sample. In some conventional sensors, an enzyme is provided that reacts with the analyte to be measured, and the byproduct of the reaction is qualified or quantified at the electrode. An enzyme has the advantage that it can be very specific to an analyte and also, when the analyte itself is not sufficiently electro-active, can be used to interact with the analyte to generate another species which is electro-active and to which the sensor can produce a desired output. In one conventional amperometric glucose oxidase-based glucose sensor, immobilized glucose oxidase catalyses the oxidation of glucose to form hydrogen peroxide, which is then quantified by amperometric measurement (for example, change in electrical current) through a polarized electrode.
One problem with electrochemical sensors is that they can electrochemically react not only with the analyte to be measured (or by-product of the enzymatic reaction with the analyte), but additionally can react with other electroactive species that are not intentionally being measured (for example, interfering species), which causes an increase in signal strength due to these “interfering species”. In other words, interfering species are compounds with an oxidation or reduction potential that overlaps with the analyte to be measured (or by product of the enzymatic reaction with the analyte). For example, in a conventional amperometric glucose oxidase-based glucose sensor wherein the sensor measures hydrogen peroxide, interfering species such as acetaminophen, ascorbate, and urate, are known to produce inaccurate signal strength when they are not properly controlled. Moreover, signal interference can result from effects, such as local ischemia, or the like, which cause the signal to produce erroneous output.
Some glucose sensors utilize a membrane system that blocks at least some interfering species, such as ascorbate and urate. In some such examples, at least one layer of the membrane assembly includes a porous structure that has a relatively impermeable matrix with a plurality of “micro holes” or pores of molecular dimensions, such that transfer through these materials is primarily due to passage of species through the pores (for example, the layer acts as a microporous barrier or sieve blocking interfering species of a particular size). In other such examples, at least one layer of the membrane assembly defines a permeability that allows selective dissolution and diffusion of species as a solute through the layer. Unfortunately, it is difficult to find membranes that are satisfactory or reliable in use, especially in vivo, which effectively block all interferants and/or interfering species.